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The economics of wastewater treatment decentralization: A techno-economic evaluation Manel Garrido-Baserba, Sergi Vinardell, Maria Molinos-Senante, Diego Rosso, and M. Poch Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01623 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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The economics of wastewater treatment decentralization: A techno-

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economic evaluation

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Manel Garrido-Baserba1,2*, Sergi Vinardell3, María Molinos-Senante4,5, Diego Rosso1,2, Manel

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Poch3

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1 Department of Civil & Environmental Engineering, University of California, Irvine, CA 92697-2175, U.S.A. (Email:[email protected])

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2

Water-Energy Nexus Center, University of California, Irvine, CA 92697-2175, U.S.A.

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3

LEQUiA, Institute of the Environment, University of Girona, E-17071, Girona, Spain

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4 Department of Hydraulic and Environmental Engineering, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile.

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5 Center for Sustainable Urban Development, CONICYT/FONDAP/15110020, Av. Vicuña Mackenna 4860, Santiago, Chile.

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*Corresponding author (T: +1-949-233-6446), E-mail: [email protected])

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ABSTRACT

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The existing wastewater treatment infrastructure has not adequately established an efficient and

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sustainable use of energy, water, and nutrients. A proposed scheme based on source separation

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and water-efficient use is compared to the current wastewater management paradigm (one

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largely based on activated sludge) using techno-economic terms. This paper explores the

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economic viability of adopting more sustainable management alternatives and expands the

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understanding of the economics of decentralization and source-separation. The feasibility of

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three different potential types of source-separation (with different levels of decentralization) are

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compared to the conventional centralized activated sludge process by using recognized

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economic assessment methodologies together with widely accepted modeling tools. The

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alternatives were evaluated for two common scenarios: new developments and retrofit due to

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the aging of existing infrastructures. The results prove that source-separated alternatives can be

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competitive options despite existing drawbacks (only when countable incomes are included),

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while the hybrid approach resulted in the least cost-effective solution. A detailed techno-

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economic evaluation of the costs of decentralization provides insight into the current constraints

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concerning the paradigm shift and the cost of existing technologic inertia.

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KEY WORDS: Activated sludge; Decentralization; Water management; Source-separation;

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Nitrogen removal

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1.

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Economic and environmental sustainability are shifting the current paradigm in urban

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wastewater management

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worldwide water sanitation significantly after increased urbanization and industrialization by

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providing safe effluent from wastewater. But this process is now being recognized as lacking

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economic and environmental sustainability, especially with respect to the inefficient use of

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energy, (recycled) water, and nutrients

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and constructed on the basis of outdated views, requirements, conditions, and technologies of

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decades ago 8. Greenhouse gas emissions from the AS process itself (e.g., N2O) and from

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sources of energy production, lack of recovery of finite nutrients such as phosphorus (even

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given current depletion rates), continually rising energy costs, resilience limitations, and the

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need for cost-efficient technologies are among the forces that will drive cities to start building

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the next generation of urban wastewater solutions 3. Furthermore, the construction of new

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developments in cities will be a worldwide phenomenon in the coming decades 9. Similarly,

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existing and aging infrastructures in wastewater treatment facilities will soon need to be

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replaced.

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In the pursuit of sustainable wastewater management options, source separation and

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decentralization are slowly becoming realistic alternatives for these new and expected

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developments10,11. Both wastewater treatment alternatives lead to several advantages:

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Introduction

-

1–4

. The century-old activated sludge (AS) process improved

5–7

. Current wastewater infrastructures were designed

The organic carbon in a typical combined municipal wastewater represents a chemical

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energy content of approximately 1.9 kWh/m3

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potential to harvest this energy within the order of 0.6 to 0.9 kWh/m3 for the most

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concentrated BW stream in households

. While anaerobic digestion has the

, the current AS process on the other hand

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consumes about 0.3-0.7 kWh/ m of wastewater 15–17.

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12–14

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The current practice is to merge, dilute, and treat both gray wastewater (GW) and black

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wastewater (BW) streams, which hinders the feasibility of nutrient recovery. Highly

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concentrated BW streams can be treated separately to facilitate nutrient recovery

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Similarly, using a minimal amount of water yields concentrated wastewater flows which

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are more cost-effective for removing harmful micropollutants

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valuable constituents 22.

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-

20,21

18,19

.

or recovering

Recovering nitrogen could reduce the production of artificial fertilizers via the Haber-

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Bosch process, which fixes nitrogen from the air but uses up to 2% of the world’s

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energy 23 and represents 50% of the energy in European agriculture8.

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-

The energy demand to run aeration blowers in the aeration-based AS process accounts

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for more than 50-75% of the net power demand in wastewater treatment plants

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(WWTPs) needed to meet the mandated amount of dissolved oxygen 24–26.

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-

Source separation and decentralization could reduce the current increase in energy

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demand (and concurrent carbon footprint) caused by the implementation of new

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technologies that achieve higher effluent quality at the expense of higher energy

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demand

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avoiding energy-demanding AS processes and transport 28–30

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-

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by producing renewable energy in useful forms (heat, methane) and by

The current trend in clean decentralized energy (i.e., biogas, solar, wind) offers new

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possibilities of decentralized wastewater treatment, making new water reuse systems

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scalable, off-grid, and without the need for the transport of fossil fuels 31–36.

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-

Vacuum toilets, as a way of source separation, can reduce BW water consumption by 90% to 35 liters per person/day 10,37–40., and the overall consumption by about 25%41.

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Increase the ability of urban wastewater systems to adapt as a response to change 42 and enhance climate-resilient infrastructures 6,43–46.

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Despite new available knowledge, expertise, and technologies to develop more economically

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and

environmentally

sustainable 47–50

water

resource

management

alternatives,

practical

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implementation remains slow

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and hardly any resources were allocated to their development because they are still considered

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immature and risky by most wastewater professionals

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that developing alternative cost-efficient wastewater management systems is an issue of

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governance rather than technology

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consumers (>64%) express highly favorable views of new systems combining elements of

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source separation, local treatment, and reduced water use 54,55. A lack of evidence pertaining to

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the economic viability of these alternatives hinders their consideration as feasible and credible

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options. Therefore, we aimed to present a clear and simple approach to the economics of source

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separation and decentralization to provide sound information that can support the decision-

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making of (waste) water authorities.

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One of the first comparison studies to date stated that depending on the scenario, source

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separated systems are more cost effective than conventional systems

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instead estimated that the overall costs of source-separated approaches could be about twice the

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conventional system

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. Source separating technologies are considered “low-tech,”

50,52,53

10,50,51

. Similarly, some authors suggest

. Furthermore, recent studies have highlighted that

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. Other authors have

. However, none of these studies used standardized and recognized

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methods of cost assessment (i.e., licensed software), and their calculations were mainly based

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on their own experience and references. Including the potential incomes from source separation

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is also essential to evaluate decentralized alternatives 58.

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The aim of this paper is to expand the understanding of the economics of decentralization and

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source-separation by using standardized approaches for economic projections and evaluations in

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wastewater systems. The feasibility of three different potential types of source-separated

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systems were compared with the AS process using commercially available modeling software

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(i.e., CapdetWorks). The main novelty of this study is to provide a comprehensive comparison

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and assessment of wastewater treatment alternatives, including the following: 1) A methodology

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based on reliable cost-estimation software (i.e., CapdetWorks; Hydromantis, Inc.) and state-of-

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the-art literature for estimating construction and operation costs; 2) Consideration of only

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existing technologies (not requiring further innovation before their deployment) that have been

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accepted as feasible alternatives among wastewater experts; 3) Inclusion of the potential income

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produced by source-separated alternatives; 4) Implementation of the aforementioned analytical

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analysis in two of the most common scenarios in developing and developed societies: New

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wastewater treatment developments and the aging of existing infrastructures (retrofit),

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respectively. Therefore, a detailed and integrated economic analysis including the sewer system,

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existing and realistic alternative wastewater treatments, and resource efficiency is presented.

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2.

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2.1 Wastewater treatment alternatives

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Two main scenarios were considered for the economic assessment of the selected wastewater

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management alternatives: new developments, and the retrofit of an existing WWTP. Both

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scenarios are evaluated for a medium-sized population of 30,000 population equivalent (PE).

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The population size was selected to represent an average, intermediate city scenario in which

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activated sludge-related configurations would typically be the preferred option to implement.

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Moreover, the installation of anaerobic digesters (AD) is often discouraged in centralized

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communities smaller than 40,000 PE due to economic and technical reasons. Phosphorus re-

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solubilization during the hydrolysis step in AD drives highly concentrated phosphorus flows

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back to the main stream, leading to recirculation instead of actual P-removal, plus increased

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piping blockage by spontaneous struvite precipitation

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without anaerobic digestion may facilitate the comparison with source-separated alternatives

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(Anaerobic-based). Note that larger treatment plants would benefit from the use of side-stream

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AD (specifically by using co-digestion strategies) and operational savings should be included in

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the case of a techno-economic analysis.

Methodology

5959–62

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. The consideration of an AS

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Figure 1. Flow diagrams for three wastewater treatment alternatives. a) Fully centralized alternative using activated sludge; b) Hybrid alternative following a centralized approach for gray water and a decentralized approach for black water; c) Fully decentralized approach for both black and gray water streams. Alternatives B and C can have two differentiated treatments for the liquid effluent from the anaerobic processes unit (Figure 2).

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Figure 2. Flow diagram alternatives for the liquid effluent from the anaerobic processes unit. Option 1) Nitrogen recovery by a stripping-absorption system; Option 2) Nitrogen removal by the Oland process.

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For each scenario, three wastewater management alternatives or flow diagrams are applied: i)

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centralized (alternative A); ii) hybrid (alternative B1 and B2) and; iii) decentralized (alternative

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C) (Figures 1 and 2; See S12 for a detailed description in supporting information). The

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centralized alternative consisted of a typical AS process. Alternative B (hybrid) is characterized

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by a partially centralized scheme, while alternative C represents a fully decentralized scheme.

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For both source-separated alternatives (B and C), the treatment of nitrogen was evaluated

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considering two different technologies. One approach was based on the physical-chemical

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recovery of the nitrogen (i.e., stripping-absorption process, alternative B1), while the other was

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based on the biological removal of nitrogen (i.e. ,Oland/anammox process, alternative B2 and

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C). For the sake of simplicity, the results of the two nitrogen treatment alternatives were shown

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only for alternative B. For the decentralized alternative (C), only the output of the most cost-

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effective alternative, i.e., nitrogen removal by the Oland process, is presented in this paper. A

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detailed description of each alternative is provided as supplemental material.

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2.2 Influent composition

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Table 1 shows the typical values for an urban influent in a centralized WWTP, which

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traditionally combines BW and GW10. The wastewater composition shown in table 1 was

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selected as the influent for all the wastewater alternatives evaluated. In the source-separation

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cases, the two different streams were split accordingly, as shown in Table 1. Following current

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practices, BW is expected to be collected with vacuum toilets, which means a consumption of

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water per people equivalent (PE) of 5 L/PE/day 63. As for GW, its water consumption is about

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108 L/PE/day 10,64.

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Table 1. Typical pollutant concentrations for the three main influents (i.e., combined, gray and

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black water) 64–69. Combined

Source-separated streams

mg/L

mg/L (GW)

mg/L (BW)

TSS - Total Suspended Solids

410.8

175.9

8,360

COD - Chemical Oxygen Demand

701.3

472.2

10,560

BOD - Biological Oxygen Demand

248.6

175.9

3,560

TKN - Total Kjeldahl Nitrogen

86.1

2.3

2,500

NH4+_N - Ammonium-N

6.1

2.3

132.4

TP - Total Phosphorus

13.7

4.6

306.0

4.3

5.9

-

-

N-NO3 - Nitrate

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Conventional toilets were used in the sewer combined alternative (alternative A, combining and

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not discerning between BW and GW), which means a consumption of about 40 L/p/day10.

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2.3 Sewer infrastructure

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For comparative and standardization purposes the sewer distribution of the new development

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(Scenario 1) and retrofit (Scenario 2) scenarios were adapted from Roefs et al. (2016). Each

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development consisted of a series of districts (See figure S4, supporting information) servicing

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1200 PE. Each district was distributed in neighborhoods of 50 households representing a total of

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120 PE. Each neighborhood had a surface area of 2.5 ha.

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In both the conventional and hybrid alternatives, districts were connected to a collection system

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that connected district to district and to the central WWTP. A backbone pipe was used as

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connection between neighborhoods within the district (Fig. S4). At the neighborhood level, both

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private and public sewers were taken into account. Private sewers were defined as the sewers

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from the house to the first Y-joint that makes connection with the water main

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description of sewer infrastructure is shown in the supplemental information.

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. A detailed

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2.4 Model domain.

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Influent variability. This study assumes low variability in the influent concentrations for the

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source-separated options as the main uncertainty contributors are avoided: Industrial effluents

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(traditionally representing 15-30% of flow composition), storm water episodes, sewage

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characteristics (i.e., combined and separated), uncontrolled infiltrations or additions, public

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spaces effluents (i.e., swimming pools, malls), etc. It is assumed that each person would produce

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a similar waste water composition (Table1) from the daily theoretical 1.2-1.5l of urine and feces

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further studies should be carried out to determine the exact impact of non-average

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concentrations.

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On the other hand, hybrid and centralized alternatives will inevitably be sensitive to load

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fluctuations that cannot be predicted with the present methodology. The reduction of uncertainty

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could be reduced further by studies incorporating wastewater dynamics and site-specific

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fluctuation.

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Sewer System. This study was based on a density of about 20 household/hectare, which is a

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common value for European residential areas with free standing or double houses

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studies that explore the variability depending on other common population densities would be

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required, as there can be lower densities (i.e., US and Canada) or higher densities (i.e., highly

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populated cities). The model used in this study was developed by Maurer et al. (2013), and may

. Nevertheless, the authors acknowledge the potential influence of influent variability and

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. Further

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yield significantly different results, especially if some pipe layouts needed to be arranged

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differently. Furthermore, the impact of assuming different distances to the central plant

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(currently 3km) may result in close to negligible results for distances shorter than 30-40km

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(following the aforementioned methodology). The methodology considers straight long-distance

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pipes (Main column or artery, HDPE) with significantly lower costs (